1. Field of the Invention
This invention relates to a method and apparatus for calcining a pulverous raw material.
2. Description of the Prior Art
Various methods and apparatus are utilized for heat treatment of pulverous raw material such as cement meal, dolomite and limestone. Calcination of cement meal requires the driving off of carbon dioxide (CO.sub.2) from calcium carbonate (CaCO.sub.3) by the endothermic process CaCO.sub.3 .fwdarw.CaO+CO.sub.2. The heat necessary to calcine and to clinker cement raw meal is usually obtained by burning fuel together with combustion air in a combustion chamber such as a rotary kiln.
In present day rotary kiln cement manufacturing plants, heat treatment of raw cement meal to accomplish evaporation of uncombined water, dehydration of clay, calcination of MgCO.sub.3, and substantial calcination of CaCO.sub.3 are carried out in a preheater having an auxiliary furnace, or calcining combustor separate from the kiln so that the size of the kiln may be reduced. Such a preheater may be termed a "precalcining" preheater because traditionally final calcination of the cement meal was carried out in the kiln.
The final heat treatment following calcination in the auxiliary furnace is a heating and sintering process by which cement clinker is produced in the rotary kiln. The clinker is discharged from the kiln onto the grate of a cooler through which air is blown to cool the clinker. The preheater usually has a plurality of stages of solids/gas separators, such as cyclone separators, plus the calcining combustor stage. The cement raw meal is initially fed into the uppermost separator stage and flows downwardly by gravity serially through the separators to the calcining combustor where fuel is burned together with combustion air at approximately 900.degree. C. to release heat energy which, together with hot waste gases from the kiln and/or hot recoup gases from the cooler, calcine the cement raw meal. The hot gases and entrained calcined meal discharged from the calcining combustor are separated in a cyclone separator and the meal flows downward by gravity to the kiln while the hot gases rise upwardly through the preceding separator stages countercurrent to the cement raw meal to preheat the raw meal. Fuel is simultaneously burned with combustion air in the rotary kiln to form a separate source of heat to achieve the relatively high temperature of approximately 1400.degree. C. and above necessary for clinkering. The vapor pressure of CO.sub.2 in a calcining combustor which receives kiln-off, or kiln waste gases raises the temperature at which calcination of raw cement meal is initiated and retards the calcination of the meal.
The following table gives the values of the pressure of CO.sub.2 corresponding to various temperatures:
______________________________________ Dissociation Pressures of CaCO.sub.3 Temperature, Pressure in Degrees C. Atmospheres ______________________________________ 500 0.000096 600 0.00242 700 0.0292 800 0.220 897 1.000 1,000 3.871 1,100 11.499 1,200 28.680 ______________________________________
The sources of carbon dioxide in a calcining combustor include the CO.sub.2 driven off from the meal in the combustor itself and that driven off from the meal in the kiln plus the CO.sub.2 present in the products of combustion of the fuel burned in the combustor and in the kiln.
A risk exists of excessively heating the raw cement meal in the calcining combustor. Temperatures of 900.degree.-1000.degree. C. are necessary for complete calcination of the meal, whereas clinkering occurs in the kiln at temperatures in the range of 1400.degree. C. and above, and burning of fuel in combustion air only can reach temperatures well in excess of 1600.degree. C. Subjection of the meal for even short periods of time to excessive temperatures can cause caking within the calcining combustor; vaporize alkali and chlorine contents in the cement meal; and produce noxious nitrogen oxides which contaminate the atmosphere into which they are discharged. When the temperature within the burning zone in the calcining combustor or in the kiln is above approximately 1100.degree. C., the alkalies and chlorine materials in the meal are vaporized. The quantity of nitrogen oxides formed increases exponentially when the temperature rises above approximately 1200.degree. C. Also sulfur in the fuel is vaporized when the fuel is burned in the combustor and in the kiln. When the temperature drops to less than 800.degree.-900.degree. C., the vaporized materials condense and adhere to the walls and block the gas inlet passage to the combustor or to the succeeding cyclone. when the vaporized alkali, sulfur and chlorine materials enter the preheater, they condense on the cement meal when the temperature drops to 800.degree.-900.degree. C. and flow downwardly with the cement meal into the kiln where they are re-volatilized and are thus contained in kiln waste gases introduced into the calcining combustor. The alkali, chlorine and sulfur material may be recirculated several times through the kiln and the preheater and build up to a concentration several times greater than their original percentages. Such alkali, chlorine, sulfur and nitrogen oxide vapors present in the calcining combustor retard the calcination of the cement meal. Further, the chlorine vapors discharged into the atmosphere may mix with water and form hydrochloric acid which creates an environmental pollution problem.
Calcining combustors of the upright, cylindrical furnace body vessel type for calcining cement raw meal separate from the rotary kiln are disclosed in such prior art U.S. Pat. Nos. as 3,869,248; 3,891,382 and 4,059,393. In certain prior art cement plants, combustion air in the form of waste gases from the kiln and recoup air from the cooler are mixed and introduced into the lower end of the calcining combustor with a swirling motion by means of a volute chamber; raw meal is fed into the combustion chamber; fuel is injected into the combustion chamber and burned with the combustion air to release heat energy to calcine the meal; and the calcined meal entrained in the upwardly swirling hot combustion gases is discharged from exhaust ducts into a cyclone separator of the preheater. In certain such prior art calcining combustors fuel is injected into combustion gases mixed with air and burns to release heat energy at a point upstream from the meal feed inlet, and such burning of combustion air and fuel within the calcining combustor can reach temperatures in excess of 1600.degree. C. and form an extremely high temperature zone which may excessively heat the meal; cause caking within the combustor; vaporize the alkali and chlorine contents of the meal; form noxious nitrogen oxides, and require use of inordinate thickness of refractory material in the walls of the calcining furnace. In other prior art calcining combustors combustion air from the cooler is mixed with the kiln-off waste gases in the calcining furnace, or before entering the furnace, and the presence of this gas impedes and delays the combustion process. Also the waste kiln-off gases will contain a high level of CO.sub.2 driven off from the meal in the kiln and from coal fired in the kiln which will raise the initial calcining temperature of CaCO.sub.3 as much as 200.degree. C. The kiln-off gases contain volatile chlorine and alkali vapors that may be condensed and form coatings on the pulverized particles that further inhibit calcination and will also coat the calcinator walls adjacent its gas inlet passage, all of which are undesirable.